Electrically conductive paper and method of making it



Sept. 964 w. A SELKE ETAI.

ELECTRICALLY coNDucTIvE PAPER AND METHOD OF MAKING IT Filed Feb. 1, I962Ala/V INVENTORS. mu/An 4. $54k: m

JO/ll a. m mm the mechanical properties 3 148 107 ELECTRICALLY CONDUCTIVE PAPER AND METHOD OF MAKING IT I William A. Selke,Stockbridge,'and John H. Mathews,

Lee, Mass., assignors to Kimberly-Clark Corporation, Neenah, Wis., acorporation of Delaware Filed Feb. 1, 1962, Ser. No. 170,395 Claims.(Cl. 162-432) v This invention relates generally to the manufactureof'paper, and has particular reference to paper having electricconductivity.

This application for patent is a continuation-in-part of the applicationfiled August 18, 1959, Serial No. 834,396, now abandoned.

It is a general object of the invention to provide a practical procedurewhereby standard paper-making machines 'and conventional paper-makingtechniques may be employed to produce, on a commercial scale, paperhaving predetermined special'conductive qualities.

Conductive paper has a wide range of uses in industry. For example, itis employed in the field of electrostatic printing and for relatedpurposes. It is also of utility in the-manufacture of transformerbushings, cable insulations, and analogous products in which, for safetypurposes, electrical stresses are to be distributed to avoid potentiallydangerous concentrations. Also, many types of heating panels andluminescent panels and similar products can employ conductive paperadvantageously.

Prior methods of imparting conductivity to paper have generally involvedincorporating in the paper either particulate conductive materials suchas carbon black, graphite, powdered metals, etc., or ionic materialssuch as salts dissolved in glycerol, quaternary ammonium compounds, andthe like. Both of these methods have serious drawbacks. The inclusion ofparticulate conductive material tends to degrade the mechanicalproperties of the paper, and the dark color of such papers limits theirsuitability for use in certain applications. Paper madeelectroconductive by the inclusion of ionic materials varies greatly inconductivity with changes in moisture content. Such papers can becomesubstantially non-conductive during periods of low relative humidity.

One object of this invention is to provide electroconductive paperswhich donot vary in conductivity when exposed to vary relative humidity.Another object is to provide electro-conductive papers which are White,or nearly white, in color. A further object is to provide electricalconductivity in paper without detracting from of the paper. A stillfurther object is to provide electrical conductivity in transparentpapers without seriously impairing their transparency. Stillanother'object of the invention is to provide electrical-conductors insheet form which are porous and permeable to Water vapor. I

The successful accomplishment of the desired objectives is predicatedupon a novel and improved manner of incorporating in the paper, inreliably controllable uniform fashion, fibrous elements having therequired electrically conductive properties and also the necessarycapability of bonding with paper fibers during usual paperformingprocedures.

In accordance with the invention a separate preliminary procedurecreates metal-coated fibers having the ability compatibly to mingle withand become adequately interengaged with the primary fibers of which thepaper is made. Glass fibers are the preferred carriers for the metalcoating; silver is the preferred metal which is deposited upon them.

One of the more particular objects of the invention is to affordsatisfactory commercially-practical solutions to the problems involvedin the silvering of fine fibers. These include the determinationandattainment of the treatment-bath proportions best suited for adequateadherence of the silver to the carrier fibers, the diameters required ofthe carrier fibers, the concentrations to be employed in the silverdepositing solution, the optimum thickness of silver upon the carrierfibers, and the most effective percentages of silvered fibers to beincorporated in the final paper.

It has been found that carrier fibers having a very small diameter and ahigh ratio of length to diameter, when coated with silver or equivalentmetal and incorporated in a paper, are admirably adapted in relativelysmall weight concentrations to provide large numbers of conductive pathsthrough the paper. A multiplicity of continuous conductive paths impartsto the paper an electrical conductivity which is fine grained anduniform. These properties are required in most cases of industrial useof such papers. To achieve this result the carrier fibers should have adiameter in the range frorn 0.05 micron to 5.5 microns, preferably from0.2 to 1.5 microns.

While other fibers can be used, glass fibers are particularly suitablefor the purpose, since glass is chemically inert and the fiber surfaceis smooth and hydrophilic.

Inasmuch as adequate conductivity can be obtained with extremely thinmetal films (less than one-millionth of an inch), only small amounts ofsilver are needed to achieve the objectives of the invention.

The accompanying figure is an enlarged schematic representation of theintermingled fibers of a paper sheet formed in accordance with thisinvention.

The silver films may be applied to the fibers by chemical reduction inaqueous media, by methods analogous to those used in the manufacture ofglass mirrors, the Brashear process being an example. However, thedeposition of silver on the surface of fine diameter glass fibers isbest effected by special techniques and by the control of processvariables within well defined limits in order to obtain products whichare useful for the present purpose. Specifically, it has been found thatthe following conditions should be observed:

(1) The ratio of fiber surface area to solution volume during thereduction of the silver complex should be at least about 1 squarecentimeter of fiber surface per cubic centimeter of solution volume, andis preferably at least about square centimeters per cubic centimeter.

(2) The concentration of silver in the silvering solution must be atleast about .0005 mol of silver per liter of solution, and is preferablyat least about .007 mol per liter.

(3) The weight of coating deposited must be at least about 5X10 grams ofmetal per cm. of fiber surface, and preferably at least about 10 gramsper cm. The preferred range of coating weight is from 10 to 5x10 gramsper cm.'. Coatings heavier than 5 X 10 grams per cm. may be applied buteconomic considerations dictate that the weight of silver be at theminimum required by the end use intended.

The following example illustrates the silvering of glass fibers inaccordance with this invention.

EXAMPLE 1 glass fibers with an average silver coating. The silveredfibers produced were electrically conductive, very dark gray in color,and were found to be coated with an amount of silver equal to about 810' grams per cm? of fiber surface.

Fibers .silvered in accordance with this process are not bright andmetallic in appearance but vary in color from a dull, almost black, tovery light gray or buff with a soft satin sheen, depending upon thethickness of the. coating and the conditions of application. Lightcolored fibers prepared by this process can be incorporated in whitepapers with very little effect on the appearance of the sheet. Table Iillustrates the effect, on the color of the silver deposit, of variationin surface-tovolume ratio, concentration of metal in the silversolutron, and weight of coating deposited.

Table 1 Silver Approximate Approximate Cone. Coating Bright- Suriace to(mols/ Weight (gms. ness Color Volume Ratio liter) of metal per cm?)0005 1. X10- 14 Very dark gray.

0008 8 10- 14 D0. 007 3 l0 17 Dark Gray. 018 3 10- 26 Medium gray. 023 310- 46 Very light gray. 035 3 l0- 53 D0. 01 ()XlO- Black 1 Percent lightreflectance in comparison with a standard magnesiadise, measured with aBausch dz Lomb opacity meter.

. The primary fibers of the paper may be natural cellulosic fibers,synthetic fibers, or mixtures. Generally, in order to insure continuityof conductive paths along the main plane of the sheet, about 1% byweight, or more, of the conductive fibers are required in the sheet.However, for certain uses of the paper, conductivity along the plane ofthe sheet may not be necessary, or may be of secondary importance; thatis, conductivity only in the direction transverse to the main plane ofthe sheet may be the primary consideration. In that case, as little as0.3% by weight of conductive fibers is usually sufficient.

The following examples show the results of incorporating the conductivefibers into various types of paper:

EXAMPLE 2 Glass fibers having an average diameter in the range of 0.2 to0.5 micron were coated with silver by the process described, the weightof coating being equal to about 1.5x grams per cm. of fiber surface.Five parts by weight of these conductive fibers were mixed with 195parts of well-beaten kraft wood pulp in an otherwise conventionalpaper-making slurry and formed into paper by conventional paper-makingprocedures. The finished product weighed 30 lbs. per 3000 square feet,contained about 2% by weight of the silvered fibers in uniformdispersion, and had a resistance along the main plane of the sheet of5x10 ohms per square.

EXAMPLE 3 Glass fibers having an average diameter in the range of 0.2 to0.5 micron were coated with silver by the process described, the weightof silver deposited being equal to about 5X10" grams per cm? of fibersurface. One part by weight of these fibers was mixed with 20 parts byweight of well-beaten kraft wood pulp and 'formed into paper byconventional paper-making proce- EXAMPLE 4 finished product weighed 30lbs. per 3000 square feet,-

contained about lV2% by weight of the silvered fibers in uniformdispersion, and had a resistance along the main plane of the sheet of2x10 ohms per square. This paper was nearly white in color and entirelyconventional in texture and appearance.

EXAMPLE 5 Glass fibers having an average diameter in the range of 0.2 to0.5 micron were coated with silver by the process described, the weightof silver deposited being equal to about 4 10- grams per cm. of fibersurface. Three parts by weight of these fiberswere mixed with 40 partsof an acrylic fiber (Orlon, one denier per filament, 1 4 inch cut). Thefibers were formed into a sheet on the wire of a' paper machine, bondedwith 20 parts of an acrylic resin, and dried. The final product was avery open, transparent paper weighing 9 lbs. per 3000 square feet. Thispaper contained about 5% by weight of the silvered fibers in uniformdispersion, and had a resistance along the main plane of the sheet of 10ohms per square.

EXAMPLE 6 Glass fibers having an average diameter in the range of 0.2 to0.5 micron were coated with silver by the process described, the weightof silver deposited being equal to about 5X10 grams of silver per cm? offiber surface; Three parts of these fibers were mixed with 50 parts ofwell-beaten wood pulp and formed into a sheet. The wet conductive sheetwas couched from the forming wire of the paper machine to a strong woodpulp paper weighing 22 lbs. per 3000 square feet and the laminated sheetwas dried. The conductive sheet weighed 7 /2 lbs. per 3000 square feet,contained about 5% by weight of the silvered fibers in uniformdispersion, and the conductive side of the laminated product had alateral resistance of 20 ohms per square.

This example is illustrative of the economical procedure that can befollowed where transverse conductivity through the sheet is notrequired. Since silver is an expensive metal it is desirable to maintainthe basis weight of the conductive paper as low as possible, thusminimizing thecost per unit area. Since very light weight papers can bemade which are completely satisfactory from the standpoint of electricalconductivity, but inadequate in mechanical strength, they can belaminated or paper-bonded to a heavier and stronger non-conductive paperas described in this Example. With papers made from natural c'ellulosicfibers the lamination can be accomplished by simply pressing the wetconductive layer onto the supporting paper, adhesion being provided bythe inherent bonding ability of natural cellulose.

EXAMPLE 7 Glass fibers having an average diameter in the range of 0.2 to0.5 micron were coated with silver by the process described, the weightof silver deposited being equal to about grams of silver per cm. offiber surface. These fibers were mixed with well-beaten wood pulp andformed into paper by conventional paper-making procedures, theproportions of metal-coated glass fibers and wood pulp, and thepaper-making process, being so controlled as to give a final productthat weighed lbs. per 2000 square feet, contained about 1% by weight ofthe metal coated fibers, and had electrical resistance along the mainplane of the sheet of about 2 10 ohms per square.

EXAMPLE 8 Glass fibers having an average diameter in the range of 0.2 to0.5 micron were coated with silver by the process described, the weightof silver deposited being equal to about 10 grams of silver per cm. offiber surface. These fibers were mixed with well-beaten Wood pulp andformed into paper by conventional paper-making pro cedures, theproportions of metal-coated glass fibers and wood pulp, and thepaper-making process-being so controlled as to give a final product thatweighed 5 lbs. per 2000 square feet, and contained about 0.3% by weightof the metal-coated glass fibers. This paper had a resistance of 2x10ohms in a direction perpendicular to the main plane of the sheet whenplaced between electrodes 6 cm. in diameter with an electrode pressureof 71 grams per cm.".

EXAMPLE 9 Glass fibers having an average diameter in the range of 0.05to 0.1 micron were coated with silver by the process-described, theweight of silver deposited being equal to about 6 10* grams of metal percm? of fiber surface. These fibers were mixed with an aqueous slurry ofwell-beaten kraft wood'pulp and formed into paper by conventionalpaper-making procedures, the proportions of metal-coated glass fibersand wood pulp, and the papermaking process, being so controlled as togive a final product containing about 6% by weight of uniformlydistributed metal-coated fibers and weighing 9 lbs. per 2000 squarefeet. This paper was found to have electrical resistance alongthe mainplane of the sheet of about 30 ohms per square.

EXAMPLE 10 Glass fibers having an average diameter in the range of 4.5to 5.5 microns were coated with silver by the process described, theweight of silver deposited being equal to about 2 10* grams of metalper'cm. of fiber surface. These fibers were mixed with an aqueous slurryof wellbeaten kraft wood pulp and formed into paper by conventionalpaper-making procedures, the proportions of metalcoated glass fibers andwood pulp, and the paper-making process, being so controlled as to givea final product containing about 16% by weight of uniformly distributedmetal-coated fibers and weighing 12 lbs. per 2000 square feet. Thispaper was found to-have electrical resistance along the main plane ofthe sheet of about 28000 ohms per square.

EXAMPLE 11 Glass fibers having an average diameter in the range of 0.75to 1.6 microns were coated with silver by the process described. theweight of silver deposited being equal to about 10- grams of metal percm. of fiber surface. These fibers were mixed with an aqueous slurry ofwellbeaten kraft wood pulp and formed into paper by conventionalpaper-making procedures, the proportions of metal-coated glass fibersand wood pulp, and the paper making process, being so controlled as togive a final product containing about 8% by weight of uniformlydistributed metal-coated fibers and weighing 10 lbs. per 2000 squarefeet. This paper was found to have electrical resistance along the mainplane of the sheet of about 200 ohms per square.

Conductive paper formed in accordance with the techniques of thisinvention has many advantages. For example, it is just as porous andpermeable as conventional papers, hence it can be utilized to advantagein the manufacture of cable coverings, transformer bushings, etc., whereits permeability facilitates the drying procedure preparatory toimpregnation with dielectric oils or resins. Also, it can be made nearlywhite in color, which gives excellent printing contrast When it is usedin electrostatic printing processes.- Moreover, its conductivity isunaffected by changes in atmospheric humidity. Another advantage stemsfrom the fine diameter of the conductive fibers, and their high ratio oflength to diameter, since it is possible .to provide a fine grainpattern of continuous conductive paths and still maintain the fiberssufiiciently spaced to achieve a high degree of transparency.Accordingly, the paper can be used as the conductive layer adjacent tothe phosphor in luminescent lighting panels, where optical transparencyis required. Other useful applications of the improved paper arenumerous, as will be readily recognized.

In many respects, ofcourse, the details herein described may be modifiedby those skilled in the art without necessarily departing from thespirit and scope of the invention as expressed in the appended claims.

What is claimed is:

1. An electrically conductive paper formed of a major proportion ofuncoated non-conductive fibers and a minor proportion of electricallyconductive metal-coated fibers composed of chemically inert materialhaving a smooth hydrophilic surface and having diameters in the rangefrom 0.05 to 5.5 microns, the amount of metal deposited upon said coatedfibers being at least 5X10 grams per square centimeter of fiber surface,said coated fibers being uniformly distributed throughout the sheet inan amount equal to at least 0.3% by weight of said sheet.

2. An electrically conductive paper as defined in claim 1, the coatedfibers being glass and the metal being silver.

3. An electrically conductive paper as defined in claim 1, said coatedfibers having diameters, before coating, in the range from 0.2 to 1.5microns.

4. An electrically conductive paper as defined in claim 1, the amount ofmetal deposited upon said coated fibers being in the range from 10- to5x 10" grams per square centimeter of fiber surface.

5. An electrically conductive paper comprising paperbonded layers ofwhich at least one is an electrically conductive sheet as defined inclaim 1, the other layer or layers being nonconductive.

6. A method of making an electrically conductive paper which consists inproviding a main mass of non-conductive paper-making fibers, separatelyproviding other fibers I composed of a chemically inert material havinga smooth hydrophilic surface and having diameters in the range from 0.05to 5.5 microns, coating said other fibers with a metal having electricconductivity'so as to make the fibers electrically conductive, saidmetal being deposited on the fibers to the extent of at least 5 X 10-grams thereof per square centimeter of fiber surface, forming an aqueousslurry containing a mixture of said main mass of fibers and said coatedfibers, and converting said slurry into a paper sheet in which thecoated fibers are uniformly distributed, the coated fibers beingemployed in an amount sufiicient to provide at least 0.3% by weight ofsaid coated fibers in said sheet.

7. The procedure defined in claim 6, in which said metal is silver andthe fibers coated with it are glass, the

silver being deposited on the fibers by reduction of a silver complex ina solution in which the fibers are immersed,

the concentration of silver in the solution being at least 0.0005 molper liter.

8. The procedure defined in claim 7, in which the ratio of fiber surfaceto solution volume is at least 1 square centimeter per cubic centimeter.

9. The procedure defined in claim 6, in which said metal is silver andthe fibers coated with it are glass, the silver being deposited on thefibers by reduction of a silver complex in a solution, in which thefibers are immersed, the concentration of silver in the solution beingat least 0.007 mol per liter and the ratio offiber surface to solutionvolume being at least 100 square centimeters per cubic centimeter.

10. A method of making an electrically conductive 8 paper which consistsin forming an electrically conductive sheet in accordance with theprocedure set forth in claim 6, and paper-bonding said sheet to at leastone other paper layer having no conductivity.

References Cited in the file of this patent UNITED STATES PATENTS2,721,139 Arledter Oct. 18, 1955 10 2,900,274 Whitehurst Aug, 18, 19593,022,213 Pattilloch Feb. 20. 1962

1. AN ELECTRICALLY CONDUCTIVE PAPER FORMED OF A MAJOR PROPORTION OFUNCOATED NON-CONDUCTIVE FIBERS AND A MINOR PROPORTION OF ELECTRICALLYCONDUCTIVE METAL-COATED FIBERS COMPOSED OF CHEMICALLY INERT MATERIALAVING A SMOOTH HYDROPHILIC SURFACE AND HAVING DIAMETERS IN THE RANGEFROM 0.05 TO 5.5 MICRONS, THE AMOUNT OF METAL DEPOSITED UPON SAID COATEDFIBERS BEING AT LEAST 5X10**6 GRAMS PER SQUARE CENTIMETER OF FIBERSURFACE, SAID COATED FIBERS BEING UNIFORMLY DISTRIBUTED THOUGHOUT THESHEET IN AN AMOUNT EQUAL TO AT LEAST 0.3% BY WEIGHT OF SAID SHEET.
 6. AMETHOD OF MAKING AN ELECTRICALLY CONDUCTIVE PAPER WHICH CONSISTS INPROVIDING A MAIN MASS OF NON-CONDUCTIVE PAPER-MAKING FIBERS, SEPARATELYPROVIDING OTHER FIBERS COMPOSED OF A CHEMICALLY INERT MATERIAL HAVING ASMOOTH HYDROPHILIC SURFACE AND HAVING DIAMETERS IN THE RANGE FROM 0.05TO 5.5 MICRONS, COATING SAID OTHER FIBERS WITH A METAL HAVING ELECTRICCONDUCTIVITY SOAS TO MAKE THE FIBERS ELECTRICALLY CONDUCTIVE, SAID METALBEING DEPOSITED ON THE FIBERS TO THE EXTENT OF AT LEAST 5X10**6 GRAMSTHEREOF PER SQUARE CENTIMETER OF FIBER SURFACE, FORMING AN AQUEOUSSLURRY CONTAINING A MIXTURE OF SAID MAIN MASS OF FIBERS AND SAID COATEDFIBERS, AND CONVERTING SAID SLURRY INTO A PAPER SHEET IN WHICH THECOATED FIBERS ARE UNIFORMLY DISTRIBUTED, THE COATED FIBERS BEINGEMPLOYED IN AN AMOUNT SUFFICIENT TO PROVIDE AT LEAST 0.3% BY WEIGHT OFSAID COATED FIBERS IN SAID SHEET.